Hedgehog (Hh) signaling was first identified in Drosophila as an important regulatory mechanism for embryonic pattern formation, or the process by which embryonic cells form ordered spatial arrangements of differentiated tissues. (Nusslein-Volhard et al. (1980) Nature 287, 795-801) In mammalian cells, three Hedgehog genes, Sonic Hedgehog (Shh), India Hedgehog (Ihh) and Desert Hedgehog (Dhh), have been identified. Hedgehog genes encode secreted proteins, which undergo post-translational modifications, including autocatalytic cleavage and lipid modification (palmitoylation) at the N-terminus and cholesterol modification of the C-terminus.
The lipid-modified N-terminal Hedgehog protein triggers the signaling activity of the protein pathway, and cell to cell communication is engendered by the dispatch of soluble Hedgehog protein from a signaling cell and receipt by a responding cell. In responding cells, the 12-pass transmembrane receptor Patched (Ptch) acts as negative regulator of Hh signaling and the 7-pass transmembrane protein Smoothened (Smo) acts as a positive regulator of Hh signaling. At resting state, free Ptch (i.e., unbound by Hh) substoichiometrically suppresses pathway activity induced by Smo (Taipale et al. (2002) Nature 418: 892); upon binding ligand Hh protein, however, repression of Smo is relieved, and the resulting signaling cascade leads to the activation and nuclear translocation of Gli transcription factors (Gli1, Gli2 and Gli3).
Downstream target genes of Hh signaling transcription include Wnts, TGFβ, and Gli1, which are elements of the positive and negative regulatory feedback loop. Several cell-cycle and proliferation regulatory genes, such as c-myc, cyclin D and E are also among the target genes of Hh signaling.
Hh signaling is known to regulate a diverse range of biological processes, such as cellular proliferation, differentiation, and organ formation in a tissue specific and dose dependent manner. In the development of neural tubes, Shh is expressed in the floorplate and directs the differentiation of specific subtypes of neurons, including motor and dopaminergic neurons. Hh is also known to regulate the proliferation of neuronal progenitor cells, such as cerebella granule cells and neural stem cells. In the developing intestinal tract, a low-level of Hh signaling is required for pancreatic development, while a high-level of Hh signaling blocks pancreatic organogenesis. Hh is also known to play important roles in stem cell proliferation and organogenesis in skin, prostate, testis and bone marrow.
Normally, Hh signaling is strictly controlled during cellular proliferation, differentiation and embryonic pattern formation. However, aberrant activity of the Hedgehog signaling pathway, due to mutations that constitutively activate the pathway, for instance, may have pathological consequences. By way of example, loss-of-function mutations of Patched are found in Gorlin's syndrome (a hereditary syndrome with high risk of skin and brain cancers, also known as Basal Cell Nevus Syndrome (BCNS)); and gain-of-function mutations of Smo and Gli are linked to basal cell carcinoma and glioblastoma. Basal cell carcinoma (BCC) is the most common form of skin cancer, affecting more than 90,000 Americans each year.
Constitutive activation of Hh has been found to promote tumorigenesis in BCC, medulloblastoma (the most common childhood brain tumor), rhabdomyosarcoma, pancreatic cancer, small cell lung cancer, prostate cancer and breast cancer. Besides the roles in tumorigenesis, Hh signaling is also implicated in the metastasis of prostate cancer. Hh signaling may be involved in many additional types of tumor types and such links are expected to continue to be discovered; this is an area of active research in many cancer centers around the world.
Proliferation of these cancer cells requires Hh pathway activation, and blocking Hh signaling pathways often inhibits cancer cell proliferation. Indeed, Hh antagonist cyclopamine and anti-Gli1 siRNA can effectively block the proliferation of these cancer cells, and can reduce tumor size in Xenograft models, suggesting that Hh antagonists, alone or in combination with other agents, could provide new chemotherapeutic regimens for the treatment of these cancers. Hh antagonist cyclopamine has been shown to suppress the metastasis of prostate cancer in animal models.
Evidence that constitutive activation of Smo results in cancers (e.g., BCC), and that Smo may be oncogenic upon its release from inhibition by Ptch, suggests utility of Smo antagonists as therapeutic agents in the treatment of such disorders. (Stone et al. (1996) Nature 384: 129). Accordingly, molecules that modulate the activity of the Hedgehog signaling pathway, e.g., which modulate Smo activity, are therapeutically useful.